Semiconductor laser device and method of manufacturing the same

ABSTRACT

There is provided a long-life and highly reliable air ridge type semiconductor laser device having a multi-layer structure including a first cladding layer of n-Al 0.7 GaInP, an active layer of AlGaAs, a second cladding layer of p-Al 0.7 GaInP and a contact (capping) layer of p-GaAs epitaxially grown in this sequential order on a substrate of n-GaAs, in which the contact layer and an upper portion of the second cladding layer is etched to be a ridge stripe, and the second cladding layer comprises an upper layer, which constitutes the ridge stripe together with the contact layer, and a lower layer having a thickness of 0.3 μm, which is positioned below the upper layer and extend outwardly from the both lower ends of the upper layer, and further, the semiconductor laser device includes a protection layer being an epitaxially grown n-GaAs layer having a film thickness of 0.15 μm provided on an upper surface of the lower layer of the second cladding layer and side planes of the ridge stripe other than an upper surface thereof.

CROSS REFERENCES TO RELATED APPLICATIONS

[0001] The present document is based on Japanese Priority Document JP2000-279552, filed in the Japanese Patent Office on Sep. 14, 2000, theentire contents of which being incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an air ridge type semiconductorlaser device and a manufacturing method of the semiconductor laserdevice. In particular, the present invention relates to a long-life airridge type semiconductor laser device having an excellent characteristicin relation between injection current and light output and amanufacturing method thereof.

[0004] 2. Description of the Related Art

[0005] There are a variety of structures for a waveguide structure of asemiconductor laser device. Among those known structures, an air ridgewaveguide type semiconductor laser device, which is often compared withthat of an embedded waveguide type semiconductor laser device, has beenfocused as a waveguide structure which can be easily fabricated. Such anair ridge waveguide type semiconductor laser device (hereinafter, simplyreferred to as an “air ridge type semiconductor laser device”) can befabricated in a following manner. First, an upper portion of an uppercladding layer is etched to form a ridge stripe. Due to a thickness of alower portion of the upper cladding layer, which is left to elongatedlyextend outwardly from a lower end edge, a predetermined difference inrefractive index in a lateral (horizontal) direction is provided.Thereby, a waveguide having an optical confinement structure in thelateral direction can be realized. The air ridge type semiconductorlaser device has advantages in that it is easy to form an opticalwaveguide, and that it requires less operational current because it isbased on real refractive index waveguide which brings small internalloss.

[0006] Now, with reference to FIG. 6, an AlGaInP-containing air ridgetype semiconductor laser device is taken as an example to explain astructure of a conventional air ridge type semiconductor laser device.FIG. 6 is a cross sectional view showing the structure of theconventional air ridge type semiconductor laser device. The conventionalair ridge type semiconductor laser device 50 has a multi-layer structureincluding an n-GaAs substrate 52, and epitaxially grown layers areformed thereon in the sequential order of a first cladding layer (lowercladding layer) 54 of n-AlGaInP, an active layer 56 of GaInP, a secondcladding layer (upper cladding layer) 58 of p-AlGaInP, an intermediatelayer 59 of p-GaInP and a contact (capping) layer 60 of p-GaAs.

[0007] In the multi-layer structure, the contact layer 60, theintermediate layer 59 of p-GaInP and an upper portion of the secondcladding layer 58 are etched to be a ridge stripe 62.

[0008] In other words, the second cladding layer 58 comprises an upperlayer 58 a which, together with the contact layer 60 and theintermediated layer 59, forms the ridge stripe 62, and a thin lowerlayer 58 b which is positioned below the upper layer 58 a andelongatedly extends from both bottom ends of the upper layer 58 aoutwardly. A dielectric film, for example, an SiO₂ film is stacked as aprotection layer 64 on an upper surface of the lower layer 58 b and sideplanes of the ridge 62. However, an upper surface of the ridge 62 to bea current injection region is not covered with the protection layer 64.In addition, a p-electrode 66 is formed on the protection layer 64 andon the contact layer 60 which is exposed from the protection layer 64,and n-electrode 68 is formed on a back surface (bottom surface in thefigure) of the GaAs substrate 52.

[0009] Such a conventional air ridge type semiconductor laser device hasa fatal problem that the period during which the device can operateexhibiting a predetermined operational property, that is, the device hasa short life. For example, there is a problem that when a fixed lightoutput is required, the longer the operating period is, the higher theoperational current becomes. In other words, if the operational currentis constant, the light output diminishes as the operating period becomeslonger. This problem has been noticeably found in air ridge typesemiconductor laser devices made of materials containing AlGaAs orAlGaInP, in particular.

[0010] As a result, it is difficult to use the air ridge typesemiconductor laser device in a field of a light source for an opticalpickup used in an optical disk recording/reproducing apparatus, forexample, which requires high reliability. Accordingly, under the presentcircumstances, the air ridge type semiconductor laser devices are usedonly in a field of a laser pointer, for example, which requiresrelatively low reliability.

[0011] Accordingly, there is a need for a high-reliable long-life airridge type semiconductor laser device.

[0012] During the study of the conventional air ridge type semiconductordevice, the Inventors have conceived that one of the factors of theshort life of the air ridge type semiconductor laser device is broughtby follows. That is, since an interface between the cladding layerexposed by etching and the dielectric film or an ohmic metal (a metalused for an ohmic electrode) at the formation of the ridge is chemicallyinstable, at the time of laser operation, operational current injectionaccelerates the deterioration of crystals in the cladding layer. Inconsideration thereof, the Inventors have hit upon an idea to have anepitaxially grown layer having a lattice constant close to that of thesecond cladding layer on the second cladding layer, as a protectionlayer, so as to chemically stabilize the second cladding layer. Aftercarrying out various experiments based on the idea, the Inventors haveinvented the semiconductor laser device and the manufacturing methodthereof, which are claimed in the application.

SUMMARY OF THE INVENTION

[0013] According to a first aspect of the present invention, there isprovided an air ridge type semiconductor laser device comprising astructure including an active layer sandwiched between a first claddinglayer (lower cladding layer) and a second cladding layer (upper claddinglayer) each having a conductivity type different from each other. Thesecond cladding layer comprises an upper layer which forms a ridgestripe, and a lower layer positioned below the upper layer, whichelongatedly extends from both lower ends of the upper layer outwardly. Aprotection layer is provided on an upper surface of the lower layer ofthe second cladding layer and side planes of the ridge stripe other thanan upper surface thereof. In the air ridge type semiconductor laserdevice, the protection layer is a compound semiconductor layerepitaxially grown on the upper surface of the lower layer of the secondcladding layer and the side planes of the ridge stripe.

[0014] According to the present invention, the epitaxially grown layeris provided as the protection layer so that crystals of the secondcladding layer are prevented from deterioration. Accordingly, theconventional problem that, when a constant light output is required, alonger operating period raises the operational current, that is, theproblem that, if the operational current is constant, the longeroperating period lowers the light output does not occur in the presentinvention.

[0015] The present invention may be applied without any limitation tomaterials for the compound semiconductor multi-layer structure whichforms the structure in which the active layer is sandwiched between thecladding layers having different conductivity types. In addition, thereis no limitation to the composition of the second cladding layer.

[0016] It is preferable that the compound semiconductor layerconstituting the protection layer is a compound semiconductor layerhaving a conductivity type different from that of the second claddinglayer. According to the arrangement above, the protection layerfunctions as a current confinement region due to p-n junctionseparation, which leads to better injection current-light outputcharacteristics.

[0017] In addition, since it is required to have the protection layerepitaxially grown on the lower layer of the second cladding layer andside planes of the ridge, in order to have the protection layer and thesecond cladding layer lattice-matched, the lattice constants of thoselayers are preferably close to each other. More preferably, thedifference between the lattice constant of the protection layer and thatof the second cladding layer is 6% or less of the lattice constant ofthe second cladding layer. For example, in a case where the structure inwhich the active layer is sandwiched between cladding layers each havinga conductive type different from each other is constituted with anAlGaInP-containing compound semiconductor, that is, in a case where thesecond cladding layer is a layer of AlGaInP, the protection layerpreferably comprises GaAs or GaInP.

[0018] Film thickness of the epitaxial compound semiconductor layerconstituting a protection layer is 0.15 μm or more and 0.3 μm or less.

[0019] The Inventors have confirmed through their experiments that theeffect of the present invention can be sufficiently achieved if theprotection layer has a thickness of 0.15 μm or more. Because it is afunction of the protection layer to stabilize a surface condition of thesecond cladding layer which has an etching surface which is chemicallyunstable on a surface thereof, a film thickness enough for stabilizingthe etching surface is sufficient for the function of the protectionlayer.

[0020] Accordingly, a film thickness which can be uniformly epitaxiallygrown by using MOCVD (Metal Organic Chemical Vapor Deposition) method orthe like is the lower limit for the protection layer. In other words,0.15 μm or more is enough for the thickness of the protection layer.

[0021] On the other hand, 0.3 μm for the upper limit of the protectionlayer is based on the following thoughts. In consideration of themanufacturing process of the semiconductor laser device, the protectionlayer is desirable to have a composition which can be epitaxially grownon the second cladding layer by the selective area growth method. Forexample, if the second cladding layer comprises AlGaInP, the protectionlayer is preferably comprised of GaAs or GaInP. In a case where thesematerials are employed in an AlGaInP-containing laser multilayerstructure, the difference between a lattice constant of the secondcladding layer and that of the protection layer can be 0.6% or less ofthe lattice constant of the second cladding layer, which is arequirement for a good epitaxial growth.

[0022] A band gap of these compound semiconductor layers is smaller thana laser oscillation wavelength of the semiconductor laser device, forexample, 650 nm in AlGaInP-containing compound semiconductor.Accordingly, the light loss due to the light absorption by the compoundsemiconductor protection layer offsets reduction of the internal losswhich is a merit of the air ridge waveguide structure. From this pointof view, an appropriate thickness of the protection layer is 0.3 μm orless. Conversely speaking, it is difficult to further improve the effectof the present invention with the protection layer having a thicknessmore than 0.3 μm.

[0023] It is preferable that the lower layer of the second claddinglayer has a thickness of 0.6 μm or less. The thickness of the lowerlayer of the second cladding layer, which forms a low refractive indexregion, is determined in accordance with the following thoughts. It hasnot been made clear how the operational characteristics of thesemiconductor laser device are deteriorated, in a narrow sense. One ofthe factors of the operational characteristics deterioration isconsidered to be a model in which a natural emitted light from an activelayer is recombined at an interfacial energy level to encourage increaseof defects. If such a consideration is correct, the lower layer of thesecond cladding layer, if only it is as thick as a cladding layer of ausual semiconductor laser device, exhibits the effect of the presentinvention.

[0024] Accordingly, since a general thickness of a cladding layer of asemiconductor laser device is within a range of about 1 μm to 2 μm, thecompound semiconductor protection layer is effectively used in asemiconductor laser device in which the lower layer of the secondcladding layer has a thickness of 2 μm or less. In particular, in a casewhere the thickness of the lower layer of the second cladding layer is0.6 μm or less, an effect of forming a refractive-index waveguide isimproved and a low-current operation peculiar to the air ridge structureis enabled so that the effectiveness of the present invention isimproved.

[0025] According to the present invention, by providing an epitaxiallygrown compound semiconductor layer as a protection layer on a surface ofthe lower layer of the second cladding layer and side planes of theridge, an operational lifetime of the air ridge type semiconductor laserdevice becomes significantly longer than a conventional air ridge typelaser device. In the semiconductor laser device according to the presentinvention, since the injection current-light output characteristics areimproved, the operational current with regard to the same light outputis lower in comparison with the conventional air ridge typesemiconductor device.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The above and other objects, features and advantages of thepresent invention will become more apparent from the followingdescription of the presently preferred exemplary embodiments of theinvention taken in conjunction with the accompanying drawings, in which:

[0027]FIG. 1 is a cross sectional view showing a constitution of an airridge type semiconductor laser device of an embodiment of the presentinvention;

[0028] Each of FIGS. 2A to 2C is a cross sectional view showing a stepof a manufacturing process of a semiconductor laser device in accordancewith a method of an embodiment of the present invention;

[0029]FIG. 3 is a cross sectional view showing a constitution of acomparative semiconductor laser device for a comparative example in anexperiment;

[0030]FIG. 4 is a graph showing a result of Experiment 1, which showsrelationship between the lapsed time after start of the operation of thecomparative semiconductor laser devices and deterioration ratecorresponding to the lapsed time;

[0031]FIG. 5 is a graph showing a result of Experiment 2, which showsrelationship between the lapsed time after start of the operation ofsample semiconductor laser devices and deterioration rate correspondingto the lapsed time; and

[0032]FIG. 6 is a cross sectional view showing a constitution of aconventional air ridge type semiconductor laser device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] Referring now to the attached drawings, embodiments of thepresent invention will be specifically and precisely described citingpractical embodiments.

[0034] <An Example of an Air Ridge Type Semiconductor Laser Device>

[0035] This example is an embodiment exemplifying the air ridge typesemiconductor laser device in accordance with the present invention.FIG. 1 is a cross sectional view showing a structure of the air ridgetype semiconductor device of the present embodiment. The semiconductorlaser device 10 of the present embodiment is an AlGaInP-containing airridge type semiconductor device. As shown in FIG. 1, on an n-GaAssubstrate 12, an epitaxially grown layers are formed thereon in thesequential order of a first cladding layer (lower cladding layer) 14 ofn-Al_(0.7)GaInP having a film thickness of 1.5 μm and a carrier densityof ×10¹⁷ cm⁻³, an active layer 16 of GaInP, a second cladding layer(upper cladding layer) 18 of p-Al_(0.7)GaInP having a film thickness of1.5 μm and a carrier density of 5×10¹⁷ cm⁻³, an intermediate layer 19 ofp-GaInP, and a contact (capping) layer 20 of p-GaAs having a filmthickness of 0.3 μm and a carrier density of 1×10¹⁹ cm⁻³ to have amulti-layer structure.

[0036] In the multi-layer structure, the contact layer 20, theintermediate layer 19 and an upper portion of the second cladding layer18 are etched to be a ridge stripe 22. In other words, the secondcladding layer 18 comprises an upper layer 18 a which constitutes theridge stripe 22 together with the contact layer 20 and the intermediatelayer 19, and a thin lower layer 18 b which is positioned below theupper layer 18 a and elongatedly extends from both lower edges of theupper layer 18 a of the second cladding layer 18. In the presentembodiment, the thickness “t” of the lower layer 18 b of the secondcladding layer 18 (see FIG. 1) is 0.3 μm.

[0037] An epitaxially grown protection layer 24 of n-GaAs having a filmthickness of 0.15 μm and a carrier density of 5×10¹⁷ cm⁻³ is provided onan upper surface of the lower layer 18 b and side planes, other than anupper surface to be a current injection region, of the ridge stripe 22.

[0038] A metal film which has an ohmic contact with the contact layer,for example, a multi-layer metal film of Ti/Pt/Au, is formed as ap-electrode 26 on the protection layer 24 and on the contact layer 20 ata portion exposed from the protection layer 24. On a back surface(bottom surface in the figure) of the n-GaAs substrate 12, an ohmiccontact metal film, such as a multi-layer metal film of AuGe/Ni/Au, isformed as an n-electrode 28.

[0039] <An Example of a Manufacturing Method of a Semiconductor LaserDevice>

[0040] This example is an embodiment exemplifying the air ridge typesemiconductor laser device 10 to which a manufacturing method of asemiconductor laser device in accordance with the present invention isapplied. Each of FIG. 2A to FIG. 2C shows a cross sectional view of eachof the steps for fabricating the semiconductor device in accordance withthe method of the present embodiment. First, as shown in FIG. 2A, on asurface of the n-GaAs substrate 12, the first cladding layer 14 ofn-Al_(0.7)GaInP having a film thickness of 1.5 μm and a carrier densityof 5×10¹⁷ cm⁻³, the active layer 16 of GaInP, the second cladding layer18 of p-Al_(0.7)GaInP having a film thickness of 1.5 μm and a carrierdensity of 5×10¹⁷ cm⁻³, the intermediate layer 19 of p-GaInP and thecontact layer 20 of p-GaAs having a film thickness of 0.3 μm and acarrier density of 1×10¹⁹ cm⁻³ are epitaxially grown in this sequentialorder, using the MOCVD method or the like, to have a multi-layerstructure.

[0041] Subsequently, as shown in FIG. 2B, an SiN film is formed on thecontact layer 20 and is patterned to form an etching mask 30. Then, thecontact layer 20, the intermediate layer 19 and the upper portion of thesecond cladding layer 18 are etched to form the ridge stripe 22. At thistime, the lower portion of the second cladding layer 18 to be the lowerlayer 18 b is not etched off. According to the procedure, the secondcladding layer 18 is processed to be the upper layer 18 a formed to be apart of the ridge stripe 22 and the lower layer 18 b having a thicknessof 0.3 μm, which is positioned below the ridge stripe 22 and extendsoutwardly from the both lower ends of the ridge stripe 22.

[0042] Next, as shown in FIG. 2C, using the etching mask 30 as a maskfor the selective area growth method, the n-GaAs layer having a filmthickness of 0.15 μm and a carrier density of 5×10¹⁷ cm⁻³ is epitaxiallygrown as the protection layer 24 by the MOCVD method on the side planesof the ridge stripe 22 and an upper surface of the lower layer 18 b ofthe second cladding layer 18.

[0043] Subsequently, the etching mask 30 is removed, and the p-electrode26 comprised of an ohmic contact metal film, for example, a multi-layermetal film of Ti/Pt/Au is formed on the protection layer and on thecontact layer 20 at the portion exposed from the protection layer 24. Onthe backside (rear surface) of the n-GaAs substrate 12, the n-electrode28 comprised of an ohmic contact metal film, for example, a multi-layermetal film of AuGe/Ni/Au is formed. In this way, the semiconductor laserdevice 10 as shown in FIG. 1 can be formed.

[0044] Now, in order to evaluate performance of semiconductor laserdevice 10 of the present embodiment, the following operationalexperiments were carried out.

[0045] <Experiment 1>

[0046] First, as a comparative example, a semiconductor laser devicehaving the same oscillator structure but no protection layer 24 asprovided in the semiconductor laser device 10, that is, as shown in FIG.3, a semiconductor laser device having a p-electrode formed directly ona surface of a lower layer of a second cladding layer and a ridge(hereinafter, referred to as a “comparative semiconductor laser device”)is subject to the operational experiment.

[0047] As shown in FIG. 3, the comparative semiconductor laser device 40has a multi-layer structure in which epitaxially grown layers includinga first cladding layer of n-Al_(0.7)GaInP, an active layer 16 of GaInP,a second cladding layer 18 of p-Al_(0.7)GaInP, an intermediated layer 19of p-GaInP and a contact (capping) layer 20 of p-GaAs are formed in thisorder on a substrate 12 of n-GaAs, as in the case of the semiconductorlaser device 10. In the multi-layer structure, the contact layer 20, theintermediate layer 19 and an upper portion of the second cladding layer18, which are to be an upper layer 18 a, are etched to form a ridgestripe 22, as in the case of the semiconductor laser device 10. In thecomparative semiconductor laser device 40, a multi-layer metal film ofTi/Pt/Au is formed as a p-electrode 26 on an upper surface of a lowerlayer 18 b of the second cladding layer 18 and the ridge 22, and amulti-layer metal film of AuGe/Ni/Au is formed as an n-electrode 28 on arear surface of the n-GaAs substrate 12.

[0048] The comparative semiconductor laser device 40 is continuouslyoperated at a constant light output, and, in accordance with the timelapsed after the start of the operation, each operational current ismeasured to calculate an increasing rate of the operational current.FIG. 4 shows the results thereof. In FIG. 4, an abscissa axis indicatesthe time lapsed after the operation start, and an ordinate axisindicates deterioration rate of every lapsed time. Herein, the term“deterioration rate” is used analogously to an increasing rate ofoperational current. For example, 10% of deterioration rate means thatan operational current at the time of the measurement is 1.1 times, thatis, 10% added to an operational current at the time the operationstarts, at a constant light output. Reference numerals 16, 17, 18, 19and 20 are numbers of samples for the comparative semiconductor laserdevice used as test samples.

[0049] As can be seen from FIG. 4, the deterioration rates of thecomparative semiconductor laser devices are remarkably high. The maximumcurrent increasing rate at the time 300 hours after the operation startis 37.12% of Sample 20, and the minimum is 12.08% of Sample 19.

[0050] In a case where the time when the deterioration rate reaches 20%is defined to be an unusable condition, a life time of the comparativesemiconductor laser device is estimated from FIG. 4 to be approximately400 hours even in the comparative semiconductor laser device showing thelowest deterioration rate, that is, Sample 19. At the time of 300 hourslapsed, the deterioration rate of Sample 20 already reaches 37%, andSamples 16, 17 and 18 are also in almost unusable states.

[0051] <Experiment 2>

[0052] Subsequently, semiconductor laser devices having the samestructure as the semiconductor laser device 10 of the present embodiment(hereinafter, referred to as a “sample semiconductor laser device”) arefabricated as Samples 11, 12, 13, 14 and 15. Each of the samplesemiconductor laser devices is continuously operated at a constant lightoutput, and, in accordance with the time lapsed after the start of theoperation, each operational current is measured to calculate anincreasing rate of the operational current. FIG. 5 shows the resultsthereof. Each of the reference numerals 11, 12, 13, 14 and 15corresponds to each of the sample numbers of the sample semiconductorlaser devices.

[0053] As can be seen from FIG. 5, the deterioration rate of the samplesemiconductor laser devices are remarkably low in comparison with thecase of the comparative semiconductor laser device. The maximum of thecurrent increasing rate at the time 500 hours after the operation startis 5.62% of Sample 15, and the minimum is 4.09% of Sample 14.

[0054] In the case where the time when the deterioration rate reaches20% is defined to be an unusable condition, as in Experiment 1, a lifetime of the sample semiconductor laser device is estimated from FIG. 5to be approximately 3,000 hours even in the sample semiconductor laserdevice showing a relatively high deterioration rate, that is, Sample 15.

[0055] <Experiment 3>

[0056] In addition, five pieces of semiconductor laser devices havingthe same structure as the conventional semiconductor laser device 50having an SiO₂ film as the protection layer 64, as shown in FIG. 6,(hereinafter, referred to as “conventional semiconductor laser devices”)are fabricated and subject to the operational experiment as in the casesof Experiments 1 and 2, described above. As a result, after theoperation start, all of the five conventional semiconductor laserdevices become impossible to oscillate before 10 hours lapses.

[0057] In view of the results of Experiments 1-3, it is exemplified thatthe life time of the semiconductor laser device 10 in which theprotection layer 24 being a semiconductor epitaxial layer provided onthe upper surface of the lower layer 18 b of the second cladding layer18 and side planes of the ridge 22 so as to protect them is dramaticallyimproved in comparison with the conventional semiconductor device andthe comparative semiconductor laser device.

[0058] In addition, since in the semiconductor laser device of thepresent embodiment, the injection current-light output characteristicsare also improved, and the operational current required is 3 mA lessthan the conventional air ridge type semiconductor laser device at thesame light output.

[0059] Furthermore, since the internal loss being small, which is acharacteristic feature of the air ridge structure, is maintained to besubstantially the same value as that of the conventional air ridge typesemiconductor laser device, as a result, the internal loss is 5 cm⁻¹less than that of the general embedded type waveguide obtained byembedding GaAs.

[0060] In the present embodiments, although the AlGaInP-containingsemiconductor laser device is taken as an example to explain the presentinvention, the present invention is not limited to the material and maybe applied to semiconductor laser devices of, for example,InP-containing, GaInP-containing, InGaAsP-containing or the like.

[0061] Although the invention has been described in its preferred formwith a certain degree of particularity, obviously many changes andvariations are possible therein. It is therefore to be understood thatthe present invention may be practiced otherwise than as specificallydescribed herein without departing from the scope and the sprit thereof.

What is claimed is:
 1. An air ridge type semiconductor laser devicecomprising a structure including an active layer sandwiched between afirst cladding layer and a second cladding layer each having aconductivity type different from each other, said second cladding layercomprising an upper layer which forms a ridge stripe and a lower layerpositioned below the upper layer, which extends outwardly from bothlower ends of the upper layer, and a protection layer provided on anupper surface of said lower layer of said second cladding layer and onside planes of said ridge stripe other than an upper surface thereof,wherein: said protection layer is a compound semiconductor layerepitaxially grown on the upper surface of said lower layer of saidsecond cladding layer and the side planes of said ridge stripe.
 2. Thesemiconductor laser device according to claim 1, wherein said compoundsemiconductor layer constituting said protection layer is a compoundsemiconductor layer having a conductivity type different from that ofsaid second cladding layer.
 3. The semiconductor laser device accordingto claim 1, wherein difference between a lattice constant of saidprotection layer and a lattice constant of said second cladding layer is6% or less of the lattice constant of said second cladding layer.
 4. Thesemiconductor laser device according to claim 1, wherein a filmthickness of said protection layer is 0.15 μm or more and 0.3 μm orless.
 5. The semiconductor laser device according to claim 1, wherein afilm thickness of said lower layer of said second cladding layer is 0.6μm or less.
 6. The semiconductor laser device according to claim 1,wherein said structure having said active layer sandwiched between saidfirst cladding layer and said second cladding layer each having aconductivity type different from each other comprises anAlGaInP-containing compound semiconductor and said protection layercomprises either one of GaAs and GaInP.
 7. A method for manufacturing anair ridge type semiconductor laser device, comprising the steps of: astep of forming a multi-layer structure by forming a first claddinglayer, an active layer, a second cladding layer and a contact layer,each comprising a compound semiconductor layer, on a compoundsemiconductor substrate, using the epitaxial growth method; a step ofetching said contact layer and said second cladding layer in saidmulti-layer structure to form a ridge stripe comprising said contactlayer and an upper layer of said second cladding layer, and a lowerlayer of said second cladding layer, which extends outwardly from bothlower ends of the upper layer; a step of epitaxially growing aprotection layer comprising a compound semiconductor layer having adifferent conductivity type from said second cladding layer on an uppersurface of said second cladding layer other than an upper surface ofsaid contact layer and on side planes of said ridge stripe using theselective area growth method; and a step of forming a metal film on anupper surface of said contact layer to form an electrode for ohmiccontact with said contact layer.